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More Nielsen Problems: Under-Counting 'Unique Visitors' by 5%

Company Making Progress in Fixing Initial Glitches, but Finding More

NEW YORK (AdAge.com) -- Nielsen disclosed today that its online measurement system has been under-counting unique visitors to the top 1,000 sites on the web by an average of 5%, the latest in a series of problems for the company.

Read Nielsen's note to clients.

In a note to clients today, the company said that social gaming sites were most affected by the latest glitch. The note did not say how long the under-counting had gone on; last week the company said problems had impacted metrics over the past three months, but sources told Ad Age that Nielsen knew about them far earlier.

Last week Nielsen disclosed it had been seriously underestimating the amount of time people spend on the web because its system was choking on long web addresses -- those with than 2,000 characters -- which have become increasingly common during visits to social networking sites like Facebook.

That was the single biggest factor in causing Nielsen to underestimate the amount of time people spend on the web by 22%. In better news today, Nielsen said it had successfully tested a fix for that problem and that the fix would be implemented for November data, which will be reported late as a result. Nielsen also said it appeared its measurement problems are largely confined to the the U.S. market.

When Nielsen first disclosed the problems last week, company execs said they were unsure as to what extent other Nielsen measurement services were affected. In addition to the 5% decrease in unique visitors, Nielsen said its video measurement service VideoCensus was affected, but not as much as the 22% decline to the Nielsen NetView numbers.

The company said VideoCensus would be fixed for the December numbers, which are released in early January, the date the company initially promised to clear up its online measurement issues.